1. Field of the Invention
This invention relates to improvements in communication secrecy systems and is particularly useful where the communication is over public communication channels.
Secrecy systems have been used for many years in military communications. Generally these systems are relatively complex and expensive. In recent time it has become desirable to have secrecy systems for law enforcement agencies and fire departments as well as other similar agencies employing public communication channels.
2. Description of the Prior Art
Secrecy systems that have been produced in the past have employed such techniques as static or rolling code band splitters to distort the voice signals to provide secrecy. Another technique that has been employed is the rearranging of the voice spectrum on a time basis. A common problem with these earlier attempts to provide secrecy in voice communications is that the system is relatively easy to break for recovery of the message. For example, in many systems employing the band spreading technique it has been found that if one band is recovered, sufficient intelligence is contained therein for recovery of the message. Similarly, if one time segment is correctly recovered with the time sharing technique, useable intelligence may be recovered. In these earlier systems the code can generally be broken without looking back at the circuitry used in scrambling the signal. Another approach that has been employed in the scrambling of the voice signals by mixing with a frequency modulated carrier frequency and the subsequent masking of the scrambled signal by application of a masking signal. However, in this system the masking signal may be removed by a simple filter in the decoding circuitry and the code broken thereafter. In addition to the problem of the codes being relatively easy to break, some of these earlier scrambling techniques have been limited to amplitude modulation communication systems and have not been useful in frequency modulated systems because of inter-modulation or cross-modulation caused by non-linearities in the communication system.